Update August 12, 2014:
At this point in the summer, we have received several reports of basil downy mildew throughout NH. If you’d like to confirm whether or not your basil has downy mildew, you may feel free to send in a photo, or a sample for confirmation, using the details below.
Original June 13 Alert:
Meg McGrath, plant pathologist at Cornell University’s Long Island Horticultural Research & Extension Center, has just reported some disconcerting news about basil downy mildew. While we have NOT seen it in NH yet, infected plants have been found in large stores in TN, WI and, just today, CT. It may be here as well, and there is a chance that gardeners may have already purchased and planted infected plants. We are looking, and are asking for your help as well. If you think you find infected basil, please send photos (email your county field specialist, and/or email@example.com and firstname.lastname@example.org) and let us know. You may also submit a sample for diagnosis using these instructions.
Basil downy mildew does not overwinter here. It comes from down South. Last year, the disease came to NH in late summer. The first symptom of the disease is leaf yellowing, similar to sunscald. In favorable conditions, you will then see blue-gray fuzzy growth on the undersides of the leaves. The leaves are not marketable or useable once infected; last year, many growers lost basil crops.
Meg McGrath’s downy mildew information page, complete with lots of photos, can be found here.
The disease first appeared in the US in 2007. Since it is a relatively new problem, there is still lots to learn about it. At this time we know that the disease may be seed borne, but that is probably a very rare event. The spores are also spread in the wind. In warm and damp conditions in mid summer spores can move far and wide.
Some types of basil are less or not susceptible at all. For example, Thai basil is resistant, but common sweet basil is highly susceptible. If you get basil downy mildew and grow more than one type of basil, please keep records and let us know which types are more resistant and susceptible.
Management tactics: Anything that will make the leaves dry quickly will help, e.g., weed control, good spacing, venting tunnels, watering in the very early morning instead of the early evening, etc. Destroy infected plants to reduce the amount of spores blowing in the wind. Making many successive plantings may increase your chance of avoiding periods of high spore movement. There are relatively few fungicides labeled for herbs, but the New England Vegetable Management Guide lists several options for managing basil downy mildew, including at least one labeled for tunnel use.
We’re very interested in tracking this disease so that we can let folks know to monitor for it, so please help us keep an eye out for it. We will keep you posted as we learn more.
Thanks to Meg McGrath (Cornell University) and Eric Sideman (MOFGA) for providing information for this note.